National Instruments AT-MIO-16 User Manual

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National Instruments AT-MIO-16 User Manual Manual pdf 39 page image

Signal Connections Chapter 3AT-MIO-16 User Manual 3-6 © National Instruments CorporationFloating Signal SourcesA floating signal source is one that is not connected in any way to the building ground systembut rather has an isolated ground-reference point. Some examples of floating signal sources areoutputs of transformers, thermocouples, battery-powered devices, optical isolator outputs, andisolation amplifiers. The ground reference of a floating signal must be tied to the AT-MIO-16analog input ground to establish a local or onboard reference for the signal. Otherwise, themeasured input signal varies or appears to float. An instrument or device that provides anisolated output falls into the floating signal source category.Ground-Referenced Signal SourcesA ground-referenced signal source is one that is connected in some way to the building systemground and is therefore already connected to a common ground point with respect to theAT-MIO-16 board, assuming that the PC AT is plugged into the same power system.Nonisolated outputs of instruments and devices that plug into the building power system fall intothis category.The difference in ground potential between two instruments connected to the same buildingpower system is typically between 1 mV and 100 mV, but can be much higher if powerdistribution circuits are not properly connected. If grounded signal source is measuredimproperly, this difference may show up as an error in the measurement. The followingconnection instructions for grounded signal sources should eliminate this ground potentialdifference from the measured signal.Input ConfigurationsYou can configure the AT-MIO-16 for one of three input modes—NRSE, RSE, or DIFF. Thefollowing sections discuss the use of single-ended and differential measurements, andconsiderations for measuring both floating and ground-referenced signal sources. Table 3-1summarizes the recommended input configuration for both types of signal sources.Table 3-1. Recommended Input Configurations for Ground-Referencedand Floating Signal SourcesType of Signal Recommended InputConfigurationGround-Referenced(nonisolated outputs,plug-in instruments)DIFFNRSEFloating(batteries, thermocouples,isolated outputs)DIFF with bias resistorsRSE